Lead frames are small metal articles that are employed to connect integrated circuits to devices in which they are used. One of the primary functions provided by the lead frame is to support the integrated circuit. The integrated circuit may be attached to the lead frame by means of a silicon-gold eutectic or silver-filled epoxy bond. The integrated circuit is also usually encapsulated in a ceramic or plastic envelope. In addition to supporting the integrated circuit, the lead frame provides connections through the envelope. These connections are used to attach the integrated circuit to the device in which it performs. The integrated circuit is electrically connected to the lead frame, usually by thermocompression or ultrasonic bonding very small gold or aluminum wires between connecting points on the integrated circuit and connecting points on the lead frame. By way of example, gold wires 1 mil in diameter may be employed, and they may be connected to the lead frame by thermocompression bonding. After being electrically connected to the lead frame, the integrated circuit is encapsulated so that leads on the lead frame are bonded to, but extend through, the ceramic or plastic capsule.
To perform its many functions, the lead frame must be made of material having many specific properties, among which are the following. A lead frame must be made of material that has good electric conductivity which is necessary to transmit electrical impulses to and from the integrated circuit. It is also necessary for a lead frame to be made of material that has good thermal conductivity because conducting heat through the lead frame is the major means of removing heat generated in the integrated circuit. Fortunately, satisfying both of these requirements is possible because thermal conductivity is directly related to electrical conductivity according to the law of Wiedmann-Franz. Good thermal conductivity is necessary in all lead frames but it is especially essential in lead frames connecting high powered integrated circuits with their environment. Lead frames must have good mechanical properties because they must support the integrated circuit and have good enough properties to maintain it in the device in which it is employed. Lead frames must have sufficient ductility to be capable of accepting precision forming, which involves the ability to be rolled to very exact thicknesses and with small tolerances. In addition, lead frames must be capable of being punched or etched to patterns with exacting dimensions. Lead frames must also be made of material that has good corrosion resistance in order to avoid being corroded during the fabrication stages of an integrated circuit. Current manufacturing technology dictates that the material of a lead frame should be easy to plate with gold or silver. For example, limited portions of lead frames are plated with gold or silver in order to make adequate electrical connections. Because the gold or silver plating is applied only to limited portions and the external leads may remain unplated, lead frames must be made of material that is itself easy to soft solder so that the integrated circuits can be readily connected to other devices. Lead frames must also be made of material that seals well with encapsulating material. Thus, the surface of the lead frame must be capable of being wetted either by the plastic that encapsulates the integrated circuit or by the ceramic material that encapsulates the integrated circuit. If a ceramic material is employed to produce a hermetic capsule, it is also essential that the lead frame have low thermal expansion so that thermal stresses during fabrication or subsequent use will not cause the hermetic seal between the lead frame metal and encapsulant to fail. A lead frame also must be made of a material that is reasonable in cost in order for its general use in inexpensie devices to be feasible.
Finding a material with all of these properties is difficult, and, as a result, compromises are made. One of the more successful materials with regard to properties is pure nickel. However, precision-rolling pure nickel to very thin strips with precise dimensions is very expensive because of high scrap losses of an intrinsically high cost material. Additionally, pure nickel has some undesirable properties for lead frames; most notably, pure nickel has high thermal expansion. An alloy of 42% nickel and the balance iron is also commonly employed in manufacture of lead frames. The disadvantages of 42% nickel alloy are high cost and a relatively low thermal conductivity which necessitates sufficient thickness of high-cost silver or gold platings to raise the thermal conductivity to adequate levels. The hardness of 42% nickel also yields it incapable of being thermocompression bonded to the interconnecting wires of the integrated circuit. Copper or copper alloys are also used; however, their corrosion resistance is inferior and may in some cases preclude their use.